The commercial utility of polyurethanes in general and aqueous dispersions of polyurethanes in particular, is due substantially to the ability of the urethane groups to undergo hydrogen bonding. Besides enhancing mechanical strength, urethane groups promote adhesion to many substrates by virtue of their ability to undergo hydrogen bonding.
Processes for making waterborne polyurethanes are well established. A review of chemical syntheses of waterborne polyurethanes and of patents and relevant publications in this area can be found in Advances in Polyurethane Science and Technology, Technomic Publishing Co., Inc., Lancaster, Pa., USA Vol. 10, pp. 121-162, the contents of which are incorporated herein by reference. Waterborne polyurethanes are obtained by first preparing a prepolymer possessing ionized or easily ionizable groups and reactive isocyanate groups. The prepolymer is produced by reacting a polyhydroxy compound such as a polyether, polyester, polycarbonate, and the like, which possesses at least two reactive hydroxyl groups with a stoichiometric excess of an aliphatic, aromatic or cycloaliphatic polyisocyanate possessing at least two reactive isocyanate groups and an organic compound possessing at least two active hydrogens and at least one ionized or easily ionizable group. The organic compound reacts with the polyhydroxy compound and polyisocyanate compound to produce an isocyanate-terminated prepolymer containing ionized or easily ionizable groups in the prepolymer backbone. In a second step, the chain length of the prepolymer is extended by the reaction of the isocyanate end groups with di- or poly- functional agents and the resulting polyurethane is dispersed in water by neutralization or removal of the ionized or easily ionizable groups.
The chain extension reaction is a crucial step. In order to obtain polyurethane dispersions possessing useful physical properties, the polymer should have optimum molecular weight. The ability of the isocyanate groups to react rather readily with water makes the chain extension step a competing reaction. Care must be taken to control the reaction course. Reactivity of the isocyanate, the chain extending agent, hydrophilicity of the polymer back bone, concentration, temperature and mechanical conditions such as rate of mixing play an important role.
One chain extension approach involves the use of aliphatic diamines which react orders of magnitude faster than water. Examples of such amines include ethylene diamine, isophorone diamine, and the like. One problem with diamine chain extenders is that diamines exhibit very high reactivity and lead to rapid build up of molecular weight, which, in turn, deleteriously affects the dispersibility of the resulting polyurethane. Another approach involves blocking the isocyanate groups with easily cleavable functionalities like oximes and later thermally unblocking them for chain extension. Initially, a good dispersion is produced and molecular weight is increased. U.S. Pat. Nos. 4,240,942, 4,387,181 and references cited therein describe such methods. However, such deblocking reactions often undesirably produce small volatile compounds such as aldehydes or ketones. Yet another approach involves using hydrazine as chain extending agent. The resulting polyurethanes are known to exhibit better mechanical properties compared to polyurethanes produced from diamine chain extended systems. However, the carcinogenicity associated with hydrazine and its derivatives limits its use in sensitive areas. Even yet another approach involves using hydrogen peroxide as chain extender. However, the resulting polyurethanes decompose at even slightly elevated temperatures. Moreover, hydrogen peroxide can only be used in water and cannot be added directly as part of the polyol composition during prepolymer formation since hydrogen peroxide is unstable in the temperature range. Also, the reactivity of hydrogen peroxide at its two terminals is the same.
Organic hydroxylamine compounds, such as aminoethanol, the aminopropanols, the aminobutanols, the aminohexanols, the aminodecanols, methylethanolamine, the amino cyclohexanols, aminobenzyl alcohol, and the like, have been disclosed as chain extenders in polyurethane syntheses. See, e.g., U.S. Pat. Nos. 2,871,227, 3,939,126, 4,066,591 and 5,155,163. However, it is believed that inorganic hydroxylamine, i.e., NH.sub.2 OH, has heretofore not been employed in the synthesis of waterborne polyurethanes. The use of inorganic hydroxylamine as chain extender results in the formation of urea-urethane linking groups which are not separated by any intervening carbon atoms. In contrast, the use of organic hydroxylamine compounds such as ethanolamine results in the formation of linking groups in which the urea groups and urethane groups are separated by carbon atoms. The urea-urethane linking groups obtained by employing inorganic hydroxylamine confer special properties on the polyurethane of this invention, as will be discussed in greater detail below.